The “Variable Coordinate” tool can be used as a stand-alone tool. However, it shows its real power in combination with the formula calculator. If a formula could already be filled with some values there, you can copy the partially filled formula and paste it into the Variable Coordinate tool. The missing values can be interpolated here.
If not all variables for the final solution of the final coordinate can or could be determined in a multi- or mystery geocache, it sometimes makes sense to simply look at all options that are still open, perhaps even visualized on a map. This way it is often possible to exclude some possibilities (of course, the possibilities should already be very manageable. It is of no use to anyone to see 1000 possible coordinates).